This invention relates to an optical signal recording and reproducing system for video discs and the like, and more particularly to such a system for reproducing correctly recorded signals not only during the recording but also after the recording.
In recent years, there have been developed record media of the type including a film of a metal such as bismuth, gold, chromium or the like attached on a substrate and capable of recording signals in the form of pits in the metal film by selectively melting or vaporizing the metal film.
Record media of the type referred to eliminates the necessity of effecting the development of latent images required for photoresist recording employed heretofore and has the advantage that recorded signals can be reproduced immediately after their recording. By putting this advantage to practical use, the recorded characteristics can be immediately observed after the recording to maintain the recorded characteristics good thereby to achieve good recording. This immediate observation means is called a "real time monitor" which is well known in the art and may be abbreviated to "RTM".
In conventional optical signal recording and reproducing systems having the function of the RTM, recording laser light from the recording light source has been modulated with a signal to be recorded and converted to a recording light beam having a suitable cross sectional profile after which the light beam is straightforwardly passed through a beam splitter and collected into a recording spot on a recording metal film such as described above by a collecting lens to form a record pit thereon. Simultaneously the linearly polarized reproducing laser light from the reproducing light source is straightforwardly passed through a monitoring polarized beam splitter and converted to a reproducing light beam having a suitable cross sectional profile after which the light beam is circularly or elliptically polarized by a quarter-wave plate and collected into a reproducing spot on the recording metallic film by the same lens as the recording light beam after its optical path has been turned by the abovementioned beam splitter. Since the recording metal film is being moved at a predetermined speed in a predetermined direction, the reproducing spot is arranged to be located in the front of the recording spot in the direction of movement of the metal and on an imaginary record track with a predetermined constant spacing between the recording and reproducing spots. In other words, the reproducing spot is temporarily delayed with respect to the recording spot but immediately reproduces a recorded signal due to the latter spot.
The reproducing light beam including the recorded signal is, as a reflected light beam, reflected from the recording metal film and moved reversely on its optical path on which the reproducing light beam has advanced to reach the metal film. As the light beam has passed forward and backward through the quarter-wave plate, the reflected light beam is linearly polarized but has a polarization plane different from that of the original laser light by an angle of 90 degrees. Therefore after having reached the monitoring polarized beam splitter, the reflected light beam advances on its optical path after having been turned through an angle of 90 degrees by the beam splitter to fall on an associated monitoring system.
Accordingly, the recorded signal can be reproduced immediately after its recording resulting in the advantages that the recorded characteristics can undergo immediate observation so as to be maintained good and therefore at good quality recorded discs can be manufactured.
However, the reproducing spot is located at a position which is fixed with respect to the recording spot so as to be stably disposed on an associated record track so that a recorded signal is satisfactorily reproduced only when the reproducing is effected simultaneously with the recording. On the other hand, playing-back previously recorded discs has been difficult in that the reproducing spots can not be stably located on the record track.
Accordingly, it is an object of the present invention to provide a new and improved optical signal recording and reproducing system for not only observing the recorded characteristics immediately during the recording to maintain the recorded characteristics at good quality but also for reproducing correctly a recorded signal which has been recorded previously.